Abstract

The kinetics of decomposition of potassium permanganate crystals have been measured over an extended temperature range, 110 to 230°C. Commercial and recrystallized samples have been used and the dependence of the kinetics on source of sample and method of preparation have been examined. By using an apparatus which measured directly the rate of reaction with improved accuracy and applying a method of empirical analysis to the results, small differences in the form of the kinetics could be observed. Reactions were carried out isothermally and with step changes of temperature. The combined results of these experiments gave the following activation energies (per mole): (a) for exponential acceleration in isothermal reactions, 37⋅0 kcal with a standard deviation of 0⋅6 kcal; (b) for reaction at an interface, 31⋅0 kcal with a standard deviation of 1⋅1 kcal; (c) for nucleation within grains approximately 50 kcal. At temperatures below 150 °C reaction was confined to the boundaries of grains. The grain size was observed by photomicrography and used to deduce the absolute rate of the interface process. It was shown that this rate was of the order predicted by absolute reaction rate theory. It was deduced from the form of the kinetics that potential nuclei within grains are on the average separated by a minimum distance related to the Burgers vector of the dislocations within the crystal. This distance was used to deduce the dislocation density and the concentration of potential nuclei. The equations of diffusion chain theory were applied to the decomposition and used to calculate the activation energy for nucleation. This theory was also used, with the additional assumption that nuclei grow equally in three dimensions, to calculate the number of nuclei present at any time during the acceleratory period. The ratio of the concentration of nuclei at the end of the exponential acceleration process to that of potential nuclei has been called the nucleation efficiency. It was shown that for isothermal reactions the nucleation efficiency decreased from about 10-1 at 220°C to about 10-4 at 140°C. From this result it was deduced that exponential acceleration comes to an end largely because of ingestion of potential nuclei by growing nuclei.

Footnotes

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